1. Field of the Invention
The present invention relates to a method for producing a semiconductor laser element, and specifically to a method for producing, with improved production yield, a high-output semiconductor laser element having an end face window structure. The present invention also relates to a semiconductor laser element which is produced by such a production method.
2. Description of the Related Art
In recent years, as a light source for an information processing apparatus which is used with an optical disk, such as a DVD (digital versatile disk), etc, a semiconductor laser element which is made by using AlGaInP mixed crystal and which emits light with a wavelength in the vicinity of 600 nm has been practically used. A rewritable optical disk such as a DVD requires an optical output of 30 mW or more. Moreover, in order to realize a smaller, faster information processing apparatus, an optical output of about 50 mW to 100 mW is required.
In general, deterioration in emission characteristics due to crystal breakage in a laser end face restricts the increase in output power of a semiconductor laser element. This is an important problem in a semiconductor laser element made using AlGaInP mixed crystal. In order to effectively increase the output power of a semiconductor laser element, the semiconductor laser element is provided with an end face window structure in which an end face of a laser cavity is made of a material transparent to laser light. An example of such an arrangement is disclosed in Suzuki, et al., xe2x80x9cElectronics lettersxe2x80x9d, Vol. 20, p. 363, 1984. This document describes an end face window structure formed by utilizing a disorder phenomenon caused in a quantum well structure. Specifically, in a double hetero structure including a quantum well structure used as an active layer, impurities (atoms) are diffused in the quantum well structure, whereby a disorder phenomenon is caused in the quantum well structure.
FIG. 16 is a perspective view showing a conventional semiconductor laser element 900. The semiconductor laser element 900 is a lateral mode controlled AlGaInP red semiconductor laser element which has an end face window structure produced by utilizing the disorder phenomenon as described above.
The semiconductor laser element 900 includes an n-type GaAs substrate 901, an n-type AlGaInP cladding layer 902, an active layer 903 which has a quantum well structure including a GaInP well layer (not shown) and an AlGaInP barrier layer (not shown), a p-type AlGaInP first cladding layer 904, a p-type GaInP etching stop layer 905, a p-type AlGaInP second cladding layer 906, a p-type GaInP band discontinuity relaxation layer 907, an n-type GaAs current confinement layer 908, a p-type GaAs contact layer 909, an n-electrode 911, and a p-electrode 912.
In the semiconductor laser element 900 having such a structure, the p-type AlGaInP second cladding layer 906 is formed so as to have a ridge shape, whereby lateral mode control of laser light is achieved. Furthermore, an impurity diffusion region 910 containing Zn atoms diffused therethrough is provided as an end face window structure of the laser element 900.
Next, a method for producing the conventional semiconductor laser element 900 is described. FIGS. 17A through 17F show steps of producing the semiconductor laser element 900. In FIG. 16 and FIGS. 17A through 17F, like reference numerals denote like parts. For the purpose of simplification, the production method is herein described for one semiconductor laser element 900, although a plurality of semiconductor laser elements 900 are produced simultaneously in an actual production process.
In the first step, by an MOVPE (Metal Organic Vapor Phase Epitaxy) method, an n-type AlGaInP cladding layer 902, an active layer 903 which has a quantum well structure including a GaInP well layer (not shown) and an AlGaInP barrier layer (not shown), a p-type AlGaInP first cladding layer 904, a p-type GaInP etching stop layer 905, a p-type AlGaInP second cladding layer 906, and a p-type GaInP band discontinuity relaxation layer 907 are sequentially formed on an n-type GaAs substrate 901, thereby obtaining a layered structure 900a having a double hetero structure as shown in FIG. 17A.
Next, an SiO2 film 913 is formed on the layered structure 900a, and the SiO2 film 913 is patterned by wet etching so as to form stripe opening portions each having a width of several tens of micrometers at the interval of several hundreds of micrometers in a direction perpendicular to a cavity direction of a resulting laser element. Then, a ZnO film 914 is formed by sputtering entirely over the SiO2 film 913 and in the stripe opening portions, and the ZnO film 914 is removed by wet-etching except for part of the ZnO film 914 which has been formed in the stripe opening portions, thereby obtaining a layered structure 900b as shown in FIG. 17B.
Then, an SiO2 film 915 is formed entirely over the upper surfaces of the SiO2 film 913 and the ZnO film 914. Thereafter, the resultant structure is annealed in a nitrogen atmosphere. In this annealing process, the ZnO film 914 formed in the stripe opening portions is used as a Zn provision layer to diffuse Zn atoms throughout the layers from the upper surface of the p-type GaInP band discontinuity relaxation layer 907 down to the n-type AlGaInP cladding layer 902. As a result, the impurity diffusion region 910 is formed, whereby a layered structure 900c is obtained as shown in FIG. 17C.
In the impurity diffusion region 910, the active layer 903 having a quantum well structure which includes the GaInP well layer (not shown) and the AlGaInP barrier layer (not shown) is disordered. In the impurity diffusion region 910, the band gap in a disordered portion of a quantum well is larger than that in a non-disordered portion, and thus, the disordered portion of the quantum well acts as an end face window structure.
Next, the SiO2 film 913, the ZnO film 914, and the SiO2 film 915 are removed by wet-etching, and an SiO2 film 916 is formed over the upper surface of the resultant structure. The SiO2 film 916 is patterned by wet-etching into a stripe shape so as to have a width of several micrometers. (As described above, in an actual production process, a plurality of semiconductor laser elements 900 are produced simultaneously, and a plurality of SiO2 films 916 are formed into a stripe pattern so that the longitudinal direction of each stripe is equal to a laser cavity direction.) The SiO2 films 916 are used as a mask to partially remove the p-type GaInP band discontinuity relaxation layer 907 by wet-etching so as to provide a ridge structure to the p-type GaInP band discontinuity relaxation layer 907. Then, the p-type AlGaInP second cladding layer 906 is etched with a wet-etching solution which can selectively etch the p-type AlGaInP second cladding layer 906, so that a ridge-shaped p-type AlGaInP second cladding layer 906 is formed. As a result, a layered structure 900d is obtained as shown in FIG. 17D. (For example, sulfuric acid may be used as the wet-etching solution for the selective etching because the etching rate thereof is different for AlGaInP and for GaInP.) In the layered structure 900d, the p-type GaInP etching stop layer 905 is exposed in the region(s) from which the p-type AlGaInP second cladding layer 906 has been completely removed.
Then, the SiO2 film 916 is also used as a mask for selective growth to grow, by an MOVPE method, an n-type GaAs current confinement layer 908 on the p-type GaInP etching stop layer 905 so as to cover side surfaces of the p-type AlGaInP second cladding layer 906 and the p-type GaInP band discontinuity relaxation layer 907 thereby obtaining a layered structure 900e as shown in FIG. 17E.
Then, the SiO2 film 916 is removed by wet-etching, and a p-type GaAs contact layer 909 is formed by an MOVPE method over the entire upper surface of the layered structure 900e from which the SiO2 film 916 has been removed. In the final step, an n-electrode 911 and a p-electrode 912 are formed, thereby obtaining a layered structure 900f as shown in FIG. 17F. In the actual production process, the resultant layered structure is cleaved along a plane in the impurity diffusion region 910 which is perpendicular to the longitudinal direction of the ridge stripes so as to obtain lager cavities each having a pair of cavity end faces. As a result, a single semiconductor laser element 900 is obtained.
Now, an operation of the above-structured conventional semiconduotor laser element 900 is described.
Referring again to FIG. 16, when the p-electrode 912 is positively biased with respect to the n-electrode 911, holes and electrons are injected from the both of the electrodes 911 and 912 toward the active layer 903. At that time, an electrical current is confined inside the ridge by the n-type GaAs current confinement layer 908 formed so as to cover the side faces of the ridge, and laser gain is obtained only inside the ridge, whereby laser oscillation is caused.
Herein, light generated in the active layer 903 is absorbed in the n-type GaAs current confinement layer 908 on the side faces of the ridge. This means that the effective refractive index of light in the active layer 903 has an imaginary part resulting from the light absorption, and the imaginary part mainly denotes a lateral distribution of light with respect to the ridge. This distribution confines laser light in a lateral direction, whereby satisfactory light beam characteristics are obtained.
Furthermore, since laser light is laterally wider in a higher mode than in a fundamental mode, in a higher mode, more leaked light is absorbed in the ridge portion, and thus, the laser oscillation in a higher mode is strongly suppressed. In such a structure, the light absorption effect is not negligible even in a fundamental mode. Therefore, a stronger suppression of light confinement and a suppression of a higher mode necessarily result in an increase of optical loss as compared with a fundamental mode. As a result, it becomes difficult to avoid an increase of a threshold current and a decrease of differential quantum efficiency.
In the above-described process for producing the conventional semiconductor laser element 900 having an end face window structure, a problem occurs in the step of selectively removing the p-type AlGaInP second cladding layer 906. As described above, etching is stopped at the p-type GaInP etching stop layer 905 except in the impurity diffusion region 910. However, in the impurity diffusion region 910, the p-type GaInP etching stop layer 905 is disordered so that mixed crystals are formed therein, and therefore, the etching rate becomes higher, whereby the etching selectivity (e.g., controllability of, etching) between AlGaInP and GaInP accordingly decreases. As a result, etching is not stopped at the p-type GaInP etching stop layer 905 but continues to the active layer 903 having the quantum well structure and the p-type AlGaInP first cladding layer 904. In the case where etching is not stopped at the p-type GaInP etching stop layer 905 in the impurity diffusion region 910, the width of the ridge differs in the impurity diffusion region 910 and in a region through which impurities are not diffused. This increases optical loss in the waveguide formed by the ridge structure, and thus increases the threshold current and the operation current. As a result, reliability of the laser element 900 significantly deteriorates.
According to one aspect of the present invention, A method for producing a semiconductor laser element includes steps of: forming a semiconductor layered structure on a first conductivity type semiconductor substrate, the semiconductor layered structure including a first conductivity type cladding layer, a quantum well active layer, and a first cladding layer of a second conductivity type; forming a diffusion control layer in a predetermined region on the semiconductor layered structure; forming a material layer which acts do an impurity source on the diffusion control layer; and diffusing impurities by a first thermal treatment from the material layer through the diffusion control layer into at least a part of the semiconductor layered structure including at least a part of the quantum well active layer, thereby forming an impurity diffusion region, wherein a part of the quantum well active layer in at least one cavity end face is disordered by diffusion of the impurities.
In one embodiment of the present invention, the semiconductor layered structure includes a double hetero structure.
In another embodiment of the present invention, the semiconductor layered structure is epitaxially grown on the first conductivity type semiconductor substrate.
In still another embodiment of the present invention, the quantum well active layer is formed between the first conductivity type cladding layer and the first cladding layer of the second conductivity type.
In still another embodiment of the present invention, the quantum well active layer has a quantum well structure including at least one well layer and a plurality of barrier layers.
In still another embodiment of the present invention, the thickness of the quantum well active layer is equal to or less than about 20 nm.
In still another embodiment of the present invention, the diffusion control layer is formed on an upper surface of the semiconductor layered structure.
In still another embodiment of the present invention, the diffusion control layer is epitaxially grown on the upper surface of the semiconductor layered structure.
In still another embodiment of the present invention, the diffusion control layer includes AlGaAs mixed crystals.
In still another embodiment of the present invention, the method for producing a semiconductor laser element further includes a step of forming a dielectric film on the material layer.
In still another embodiment of the present invention, the diffusion control layer includes a material in which a diffusion rate of the impurities are lower than in the quantum well active layer.
In still another embodiment of the present invention, the semiconductor layered structure includes AlGaInP mixed crystals.
In still another embodiment of the present invention, an amount of the impurities diffused into the semiconductor layered structure is controlled by controlling at least one of a conductivity type, a composition, and a thickness of the diffusion control layer.
In still another embodiment of the present invention, an amount of the impurities diffused into the semiconductor layered structure is controlled by controlling at least one of a temperature and a time length of the first thermal treatment.
In still another embodiment of the present invention, the diffusion control layer is of a first conductivity type.
In still another embodiment of the present invention, the diffusion control layer is of a second conductivity type.
Instill another embodiment of the present invention, the diffusion control layer is non-conductive.
In still another embodiment of the present invention, the diffusion control layer includes GaAs.
In still another embodiment of the present invention, a thickness of the diffusion control layer is from about 50 nm to about 300 nm. Preferably, the thickness of the diffusion control layer may be from about 50 nm to about 200 nm.
In still another embodiment of the present invention, a temperature of the first thermal treatment is equal to or less than about 650xc2x0 C. Preferably, the temperature of the first thermal treatment may be from about 500xc2x0 C. to about 650xc2x0 C.
In still another embodiment of the present invention, the material layer includes at least one of Zn and Mg.
In still another embodiment of the present invention, the material layer includes ZnO.
In still another embodiment of the present invention, a thickness of the material layer is equal to or less than about 50 nm. Preferably, the thickness of the material layer may be from about 10 nm to about 50 nm.
In still another embodiment of the present invention, the dielectric film includes at least one of SiO2, Al2O3, TiO2, and SiN.
In still another embodiment of the present invention, the diffusion control layer includes AlGaAs in which an AlAs composition is about 20% or more.
In still another embodiment of the present invention, the semiconductor layered structure further includes a second conductivity type contact layer.
In still another embodiment of the present invention, the second conductivity type contact layer includes AlGaAs in which an AlAs composition is about 20% or less.
In still another embodiment of the present invention, an undoped semiconductor layer is formed between the first conductivity type cladding layer and the quantum well active layer.
In still another embodiment of the present invention, a thickness of the undoped semiconductor layer is equal to or greater than about 40 nm.
In still another embodiment of the present invention, the method for producing a semiconductor laser element further includes a step of forming a low reflective coating film on the at least one cavity end face.
In still another embodiment of the present invention, the semiconductor layered structure includes: a second conductivity type etching stop layer formed on the first cladding layer of the second conductivity type; and a second cladding layer of a second conductivity type formed on the second conductivity type etching stop layer.
In still another embodiment of the present invention, the method for producing a semiconductor laser element further includes steps of: etching the second cladding layer of the second conductivity type into a ridge shape; and forming a first conductivity type current confinement layer on a side face of the second cladding layer of the second conductivity type.
In still another embodiment of the present invention, the method for producing a semiconductor laser element further includes a stop of removing the diffusion control layer, wherein after the step of removing the diffusion control layer, the second cladding layer of the second conductivity type is etched into a ridge shape.
In still another embodiment of the present invention, the conductivity type of the diffusion control layer is the first conductivity type.
In still another embodiment of the present invention, the method for producing a semiconductor laser element further includes a step of performing a second thermal treatment after the step of forming the first conductivity type current confinement layer.
In still another embodiment of the present invention, wherein: the quantum well active layer has a quantum well structure including at least one well layer and a plurality of barrier layers; and a thickness of the quantum well structure is equal to or less than about 7 nm. Preferably, the thickness of the quantum well structure may be from about 3 nm to about 7 nm.
Instill another embodiment of the present invention, a thickness of the second conductivity type etching stop layer is equal to or less than about 15 nm. Preferably, the thickness of the second conductivity type etching stop layer may be from about 3 nm to about 15 nm.
In still another embodiment of the present invention, the step of forming the material layer is performed after the step of forming the first conductivity type current confinement layer.
In still another embodiment of the present invention, in the step of diffusing impurities into at least a part of the semiconductor layered structure, impurities diffused into a part of the second cladding layer of the second conductivity type, and a concentration of the impurities diffused in a part of the second cladding layer of the second conductivity type within the impurity diffusion region is equal to or less than about 1xc3x971918 cm3.
In still another embodiment of the present invention, the semiconductor layered structure further includes a second conductivity type etching stop layer formed on the first cladding layer of the second conductivity type, the method further comprising steps of: forming a first conductivity type current confinement layer on the semiconductor layered structure: forming an opening portion in the first conductivity type current confinement layer; and forming a second cladding layer of a second conductivity type on the first conductivity type current confinement layer and the opening portion, wherein the step of forming the diffusion control layer is performed after the step of forming the second cladding layer of the second conductivity type.
According to another aspect of the present invention, a semiconductor laser element includes: a first conductivity type semiconductor substrate; and a semiconductor layered structure, wherein the semiconductor layered structure includes: a first conductivity type cladding layer; a quantum well active layer; a first cladding layer of a second conductivity type; a second conductivity type etching stop layer: a second cladding layer of a second conductivity type having a ridge shape; and a first conductivity type current confinement layer formed on a side face of the second cladding layer of the second conductivity type, wherein a part of the quantum well active layer which is positioned under the second cladding layer of the second conductivity type and on at least one cavity end face is disordered by impurities.
In one embodiment of the present invention, the semiconductor layered structure includes a double hetero structure.
In another embodiment of the present invention, the quantum well active layer is formed between the first conductivity type cladding layer and the first cladding layer of the second conductivity type.
In still another embodiment of the present invention, the quantum well active layer has a quantum well structure including at least one well layer and a plurality of barrier layers.
In still another embodiment of the present invention, the semiconductor layered structure includes AlGaInP mixed crystals.
In still another embodiment of the present invention, the semiconductor layered structure further includes a second conductivity type contact layer.
In still another embodiment of the present invention, the semiconductor laser element further includes a low reflective coating film on the at least one cavity end face.
According to still another aspect of the present invention, a semiconductor laser element includes: a first conductivity type semiconductor substrate; and a semiconductor layered structure, wherein the semiconductor layered structure includes: a first conductivity type cladding layer; a quantum well active layer; a first cladding layer of a second conductivity type; a second conductivity type etching stop layer: a first conductivity type current confinement layer having an opening portion; and a second cladding layer of a second conductivity type provided at least in the opening portion, wherein a part of the quantum well active layer which is positioned under the opening portion and on at least one cavity end face is disordered by impurities.
In one embodiment of the present invention, the semiconductor layered structure includes a double hetero structure.
In another embodiment of the present invention, the quantum well active layer is formed between the first conductivity type cladding layer and the first cladding layer of the second conductivity type.
In still another embodiment of the present invention, the quantum well active layer has a quantum well structure including at least one well layer and a plurality of barrier layers.
In still another embodiment of the present invention, the semiconductor layered structure includes AlGaInP mixed crystals.
In still another embodiment of the present invention, the semiconductor layered structure further includes a second conductivity type contact layer.
Thus, the invention described herein makes possible the advantages of (1) providing a method for diffusing impurities in which a stable etching can be performed with high repeatability, (2) suppressing optical lose in the waveguide, and (3) improving the production yield and reliability of a semiconductor laser element.